A power failure-free power supply apparatus capable of minimizing time required for parallel operation of a power rectifier and a power rectifier/inverter, to thereby minimize a deterioration in operational reliability. The power rectifier converts an output of an AC power supply into DC power. A power inverter converts DC power into AC one. The power rectifier/inverter carries out rectifying/inverting operation. An output of the power rectifier and an AC output of the power rectifier/inverter are fed through a change-over unit to a load. Operation of switches is controlled depending on a detection signal of a current detector for detecting a charging current of a storage battery. A control unit permits parallel operation of the power rectifier and power rectifier/inverter when a residual capacity of the storage battery is at an increased level and permits the storage battery to be charged when the residual capacity is at a reduced level. An increase in charging current during the parallel operation minimizes time required for the parallel operation.